Synthetic composite materials possess desirably high strength-to-weight ratios; however, synthetic composites typically have lacked dynamic functionality that occurs in natural composite materials. Natural composite materials can utilize vascular networks to accomplish a variety of biological functions, in both soft and hard tissue. For example, composite structures such as bone tissue or wood are lightweight and have high strength, yet contain extensive vasculature capable of transporting mass and energy.
Another feature of natural composite materials is their ability to have different levels of communication between distinct vascular networks, depending on the specific role of the material within an organism. In one example, the phloem and xylem channels of plant vascular bundles may be independent and separated by lignin layers, such that the contents of the channels do not interact. In another example, physical contact between the artery and vein channels of a human or animal circulatory system may provide for heat transfer between the channels; however, the chemical compositions of the fluids in these channels remain separate. In yet another example, gas exchange of oxygen and carbon dioxide occurs between the blood vessel channels and the alveoli channels within the lungs.
An ongoing challenge in materials science is the development of microvascular networks in synthetic materials using conventional manufacturing processes. Specialized fabrication methods such as laser-micromachining, soft lithography, templating with degradable sugar fibers, and incorporating hollow glass or polymeric fibers can produce some microvascular structures in synthetic materials. These specialized methods, however, are not currently suitable for rapid, large-scale production of materials having complex vasculatures.
In one approach to microfluidic materials, relatively short microfluidic channels are provided in a matrix in the form of hollow glass fibers (WO 2007/005657 to Dry). The glass fibers are present as repair conduits containing a fluid that can heal a crack in the composite matrix. A significant limitation of this approach is the brittle nature of the hollow glass fibers, which limits the shapes and lengths of microfluidic channels that can be present in the material. In addition, the glass fibers cannot readily be used to form a microfluidic network.
In another approach to microfluidic materials, microfluidic channels are formed in a polymeric matrix by arranging hollow polymeric fibers and then forming the matrix around the hollow polymeric fibers (U.S. Publication No. 2008/0003433 to Mikami). Hollow polymeric fibers may offer a wider variety of microfluidic channel shapes than those available from hollow glass fibers. This approach, however, also has a number of limitations, including an inability to form a network from the individual hollow fibers, the relatively small number of materials available as hollow fibers, and the possibility of incompatibility between the hollow fiber and the matrix.
Microfluidic networks can be formed in a polymeric matrix using a three-dimensional (3-D) direct-write assembly technique (U.S. Publication No. 2008/0305343 to Toohey et al.). While this fabrication method provides excellent spatial control, the resulting networks typically will not survive the mechanical and/or thermal stresses encountered in the conventional processes of forming reinforced composites.
It is desirable to provide multiple microvascular networks in synthetic materials, where the type and level of communication between the distinct microvascular networks can be varied between different materials. It is desirable for such complex microvascular networks to be formed using conventional manufacturing processes. It also is desirable for the microfluidic channels of the different networks within a material to be available in a variety of shapes and dimensions, and for a variety of polymers to be available as the polymeric matrix of such composites.